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So opponent is decked...
He would only ignore it if he were ignoring the character it was played by at the time it was played. Treachery is like Removing the Curse, it's always ongoing with an effect you can choose when to trigger. If it had been played during a prep or discard phase, or a battle phase when Watchful Servant's SA was not active, it would work.
The definition of cards like this are such that, while 'played' already, they are still effects being used by that character
QuoteThe definition of cards like this are such that, while 'played' already, they are still effects being used by that characterIncorrect. Your're correct about the two examples you give, but they do not follow to this conclusion. The Herod is not using Herod's Treachery, he's just the location its been placed on, whatever character the card was played off originally is the only character that used it, and if he was not Ignored at the time then HT works against Servant.